The present invention relates to a process of reducing sulfur content of a gaseous stream. More particularly, the present invention relates to a process of reducing hydrogen sulfide in a gaseous stream with the production of elemental sulfur.
The removal of sulfur components, particularly hydrogen sulfide, from gaseous streams such as the waste gases generated in the course of various chemical and industrial processes, for example, in the pulping of wood, in the production of natural gas and crude oil and in petroleum refining, has become increasingly important in combating atmospheric pollution. Gases containing sulfurs, particularly hydrogen sulfide, not only have an offensive odor, but such gases may also cause damage to vegetation, painted surfaces, and wild life as well as raising a significant health hazard to humans. Governmental regulations have increasingly imposed lower tolerances on the sulfur content of gases which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the sulfurs, particularly hydrogen sulfide.
Natural gas is a mixture of several gaseous components, the percentages of which may vary widely from one reservoir to another. The principal energy median in natural gas are hydrocarbons, with methane usually making up the largest percentage. Natural gas also contains a lesser quantity of higher hydrocarbons, such as ethane and propane, and it almost always contains some non-combustible constituents such as carbon dioxide, nitrogen, and traces of rare gases.
Some natural gases also contain hydrogen sulfide. Gas which has a hydrogen sulfide content of less than 1% is called "lean sour gas"; gas which has more is called "sour gas". Gas that does not contain any hydrogen sulfide is known as "sweet gas".
Production of sour gas reservoirs requires carefully specified process equipment, and techniques, as well subsequent purification treatment. Due to the hydrogen sulfide content in the untreated gas, strict safety precautions must be observed in transportation and processing of sour gas. Sour gas is highly corrosive, and therefore also requires the use of high-grade materials. Hydrogen sulfide is removed from natural gas in processing plants which require extremely high capital outlay on the part of the natural gas producers, plus special know-how to handle the sophisticated process technology.
Natural gas production and petroleum refinery streams are typically desulfurized by the Claus process wherein elemental sulfur is produced by reacting hydrogen sulfide and sulfur dioxide in the presence of a catalyst. The Claus process uses a combustion furnace or chamber which, at 950.degree. C. to 1,350.degree. C. (1742.degree. F.-2462.degree. F.) converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur. Thereafter the remaining gas is heated and fed to a claus reactor where the gas is reacted over a catalyst to form elemental sulfur. Typically, the gas passes through at least two such Claus catalyst stages.
The different stages of the Claus process may be represented by the following equations. EQU H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (Eq.I) EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O (Eq.II)
The overall reaction is: EQU 3H.sub.2 S+3/2O.sub.2 .revreaction.3S+3H.sub.2 O (Eq.III)
The final Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, carbon oxysulfide, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas generally is subjected to post-combustion to convert substantially all the sulfurs to SO.sub.2 and then further purified by Claus after-treatments. Such after-treatment is carried out either directly after the last catalytic process stage or before the post-combustion, depending on the type of process. These additional after-treatment installations are generally complicated and expensive with regard to apparatus and process technology due to the diversity of the sulfur compounds occurring in the Claus exhaust gas.
U.S. Pat. Nos. 4,552,746 and 4,857,297 disclose the use of titanium oxide for the direct catalytic oxidation of hydrogen sulfide in gas mixtures with air or oxygen into elemental sulfur and water. These titanium oxide catalysts require that the feed have water constraints, for example that the water content be less than 10 volume percent.
It is an object of the present invention to provide a direct catalytic oxidation process having substantially improved conversion, and water tolerance.